The present invention relates to an electronic camera having an image-pickup device that converts an image of an object into electric signals.
Recently, electronic still cameras and video cameras that electronically stores still images and animated images in memories have become popular. In these electronic cameras, a photographing lens system forms an image of the object on a light receiving surface of an image-pickup device, e.g., a CCD image sensor. The image-pickup device has a plurality of pixels arrayed in a lattice pattern, i.e., the pixels are arrayed at constant interval in both vertical and horizontal directions on the light receiving surface thereof. These pixels convert the optical image formed on the light receiving surface into electrical signals and the electrical signals obtained are further converted from analog to digital and then stored in a recording medium as image data.
Such an electronic camera often includes a low-pass filter and/or an infrared-absorbing filter between the photographing lens system and the image-pickup device.
A moiré appears in the image obtained by the image-pickup device when the image has a spatial frequency close to the spatial sampling frequency of the image-pickup device, which is defined by the interval of the pixels arrayed in the lattice pattern as mentioned above. The low-pass filter is disposed in front of the image-pickup device to attenuate the spatial frequency components close to the sampling frequency of the image-pickup device so that the moiré is prevented.
The low-pass-filter is formed as a single optical filter by sticking together a plurality of birefringent plates made from crystal or lithium niobate. The low-pass filter splits an incident ray of light into four rays, two spaced apart in the lateral direction and the other two spaced apart in the vertical direction. In other words, the low-pass filter splits the incident ray in the directions in which the pixels of the image-pickup device are arrayed. Accordingly, four identical images of the object are formed on the light receiving surface overlapping to each other. The thickness of the low-pass filter is arranged such that the spaced images are spaced apart to each other with a displacement equal to the distance between the pixels. Thus the resulting image formed on the light receiving surface is slightly blurred so that the spatial frequency of the image is attenuated in the range near to half of the sampling frequency of the image-pickup device.
The infrared-absorbing filter is disposed in the passage of the light incident on the image-pickup device to eliminate the infrared components from the light. This is necessary since the image-pickup device has a different spectral sensitivity in comparison with the human eye's and receives not only visible radiation but also infrared radiation.
The optical filters, i.e., the low-pass filter and the infrared-absorbing filter, tend to collect dust on their surfaces due to static electricity. Especially low-pass filters made from a ferroelectric crystal having a pyroelectricity, like lithium niobate, for example, collect dust easily since polarization charges are generated even under small temperature change. Such dust decreases the quality of the image formed on the image-pickup device.
The dust may also adhere on the light receiving surface of the image-pickup device due to static electricity when the filter is not affixed on the light receiving surface but disposed such that a gap exists between the filter and the light receiving surface. Such dust also decreases the quality of the image obtained by the image-pickup device.
It may be possible to remove the dust from the filters or light receiving surface of the image-pickup device by blowing air using a blower or spray, or wiping the dust off with clothes. However, it is not easy to remove the dust completely by such methods since the dust removed are pulled back to the filters and/or image-pickup sensor by the electric attraction if the filters and/or image-pickup device are electro-statically charged.